MAINTAINERS: add entry for Amlogic DRM drivers

Add myself as maintainer for Amlogic DRM drivers.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>